Vanessa Trump’s prom date for senior year at her $47,000-a-year Upper West Side prep school dressed up for the big occasion in a tuxedo, a bow tie — and a necklace in the black-and-gold colors of the Latin Kings, a street gang he had joined in an upstate prison.

Vanessa — who became part of America’s first family in 2017 as the wife of Donald Trump Jr. — was known at the Dwight School for her bright smile, model looks and outstanding ability on its tennis courts. But on weekends, she drove high school sweetheart Valentin Rivera — who has since served various prison terms for assault, weapons offenses and negligent homicide — to the Lower East Side, where he led meetings of his chapter of the gang.

And in an exclusive interview, Rivera told Page Six how Vanessa would help him drop off weed to friends by driving him around in her stepfather’s convertible with an illegal gun in the trunk.

Her school friends came to fear Rivera. “If anybody disrespected her in any way, I would physically put my hands on them,” Rivera told us. “Any types of disrespect — call out her name, mess with her, anything.”

Even their first meeting — around fifth grade at P.S. 158 at 77th Street and York Avenue, Rivera recalls — was tinged with trouble. “We was in the carnival in the school and I hit her with a water balloon, and she told on me. I went to the principal’s office,” Rivera, who grew up in a 79th Street building where his dad was the super, told us. “Then we parted ways until a couple of years later.”

Valentin RiveraAnnie Wermiel/NY Post

In those intervening years, Rivera got in trouble for fighting with neighborhood kids, and was sent to Highland Residential Center outside Poughkeepsie, NY. “I came home from juvie, then I met her again,” he says of Vanessa, who has been in the midst of a divorce from Don Jr. since March.

Vanessa and Rivera, by then about 15, ran into each other at a house party on 86th Street, he said, where Vanessa — then Vanessa Haydon, the daughter of Bonnie Haydon, who ran the Kay Models agency, and stepdaughter of power lawyer Charles Haydon — asked Rivera and his friends for help with some kids who were causing trouble.

“They were stealing out the house and we searched them,” he said. “That’s how we started talking again.”

They quickly began dating, but a year later, Rivera was arrested for assault and given a 1 1/3-to-four-year prison sentence. Before transferring to an upstate maximum-security youth prison, Rivera spent a few weeks on Rikers Island, where Vanessa — who would go on to give birth to five of the president’s grandchildren — came to visit him with Rivera’s mother.

“She was heartbroken that I was being taken away,” he said. “The first time [she came to visit], she was upset — she cried a little bit — but after a while, we get used to things. She only came to visit me twice because her mother forbid her from coming to visit me.”

Once upstate, Rivera quickly got a rep that caught the eye of the Latin Kings, a street gang that formed in Chicago in the 1940s but that is now reputed to be the largest Hispanic gang in the world. “A few fights, a few slashings, and my name was ready in the prison as a person who was a bad guy.”

Valentin Rivera and his then-girlfriend Vanessa Trump (née Haydon) at a New York City prom

Vanessa kept in touch with him in prison, and after a 16-month sentence, “I came home [to New York City], I was a full-blown Latin King, and now I came home to a whole nation of Latin Kings.”

He also came home to Vanessa. Though they were young, Rivera tells us that he and Vanessa were deeply in love and even discussed getting married and having children. She presciently told Rivera that she wanted to have five kids — which she eventually did with Don Jr.

Rivera declined to discuss the specifics of his activity in the gang, saying only, “I carried out things that needed to be carried out in the Nation.”

As well as criminal activities, the Latin Kings — whose New York arm was founded in Collins Correctional Facility in 1986 — pride themselves on social work, like feeding the homeless. Rivera says that outside prison, he rose through the ranks.

“[Vanessa] was worried in the beginning,” Rivera told us, “but after I guess seeing the power of who I became, she was cool with it.”

Vanessa regularly drove Rivera to gang meetings at locations around the city, including the El Bohio community center on the Lower East Side. She wasn’t welcome to stay for meetings since she was neither a member nor — without Latin heritage — eligible to become one.

“She was fascinated. We were kids. She liked the street life at the time,” said Rivera. “She liked that type of environment — being around gang members and stuff.”

At Dwight — where Rivera could frequently be seen at the school gates waiting to pick her up — he gave her a dangerous mystique. Asked what appealed to Vanessa about the gang lifestyle, Rivera said, “Maybe it was the attention she was getting. From her classmates,” he said, “from everybody.” (A Dwight classmate once described Vanessa to New York magazine as “an ill thug.”)

While Vanessa blended in with the uptown set, Rivera told us that after spending time around gang members, she became a little more “rugged” and even “a little cocky.”

“She got into a couple of little catfights,” Rivera told us. “She was pretty jealous,” he said, “as far as other girls and stuff, she let it be known that I was her man.”

Valentin Rivera and his then-girlfriend Vanessa Trump (née Haydon)

(This will be familiar to reality star Aubrey O’Day, who had an affair with Don Jr. in 2012 while he was married to Vanessa. A pal of O’Day’s recently told Page Six that when Vanessa discovered the affair, she called O’Day and “went gangster on her.”)

While Rivera lent Vanessa street cred and protection, her connections could be useful to him as well. Rivera says Vanessa used to drive him to deliver weed with his illegal gun stashed in the trunk of her convertible — and more than once dropped her lawyer father’s name to get out of trouble. “If we got pulled over, the police would just let us go,” he said. “She’ll give up Charles’s information. Car’s registered to Charles Haydon. They’ll just cut us loose.”

Their five-year romance ended in 1998 when Rivera discovered in the New York Post that Vanessa — by then a model with the Wilhelmina agency — had been cheating on him with Leonardo DiCaprio.

“DiCaprio’s luscious love is 20-year-old fashion model Vanessa Haydon. The passionate pair have been inseparable over the past two weeks,” Page Six revealed on May 1, 1998. They “tried to keep a tight lid on their budding romance — but it all spilled out during a private party in a trendy SoHo loft last week … The couple could be seen holding hands, kissing and cuddling into the wee hours in a dimly lit corner.”

“I don’t think it’s a fling,” an acquaintance of Vanessa told Page Six at the time.

Although he was heartbroken and humiliated, Rivera says his fellow gang members teased him about her infidelity, with two of them holding a third as if he were on the bow of a ship and singing Celine Dion’s “My Heart Will Go On,” the theme from DiCaprio’s movie “Titanic.”

Asked if Rivera ever considered retaliating against DiCaprio, he laughed. “I thought about it!”

Donald Trump Jr. and Vanessa Trump in AprilREUTERS

A week after the breakup, Rivera was arrested by the FBI on RICO charges, which he later beat. He did several more long stints in prison — including for negligent homicide because of a tragic accident in which he shot a friend while they were fooling around with a gun — but became “inactive” in the Latin Kings in 2013 and has since turned his life around. A father of one, Rivera now drives seniors to medical appointments.

Rivera — who says he’s no fan of President Trump and wasn’t thrilled to see her move on to Don Jr. — calls Vanessa his “first love,” and in spite of their bad breakup, says, “I’m happy for her that she has five beautiful children and she’s doing well.”

When reached for comment, a rep for Vanessa told us, “Vanessa is a devoted mother of five amazing children. This is nothing more than an attempt to sensationalize over20-year-old ‘stories’ from high school for clickbait.”